Activist investor Carl Icahn reduced his stake in Freeport-McMoRan Inc., the world's biggest publicly traded copper miner, to 6.33 percent as of June 6, from 7.2 percent in late November, according to a regulatory filing on Friday.

In 2015, the billionaire investor took a large stake in the Phoenix, Arizona-based company, saying Freeport was undervalued.

Last year, Freeport pledged to cut its debt in half, and is now very close to that target with "a bit over $11 billion in net debt," Chief Financial Officer Kathleen Quirk said via webcast a Deutsche Bank conference this week.

Its debt load had ballooned to $20.1 billion at the end of 2015, reflecting two big oil and natural gas acquisitions. To reduce the debt, Freeport sold assets valued at more than $6 billion in 2016.

Icahn held 91.6 million Freeport shares as of June 6, a filing with the U.S. Securities and Exchange Commission showed. Based on Freeport's closing stock price of $12.36 in New York, that holding is worth approximately $1.13 billion.

In a Nov. 23 filing, Icahn disclosed that his stake had dropped to about 7.2 percent, or 104 million shares, from an 8.8 percent holding previously.